There are many occasions when it is necessary to determine the spectrum (optical power distribution over frequency) of an optical signal (a beam of light). This measurement need extends from ultraviolet wavelengths (less than 400 nanometers), through visible wavelengths (400 to 800 nm) on into the near-infrared wavelengths (800 to 2,000 nm) often used for fiber optic telecommunications, and in the infrared wavelengths (more than 2,000 nm) often used for spectroscopic identification of materials. This can be done qualitatively at visible wavelengths by an ordinary prism that splits a beam of light into its various colors.
To obtain a quantitative indication of the various frequencies in a beam of light and of their relative strengths, the prism can be mechanically pivoted such that the various color beams emanating from the prism are sequentially focused onto a photodetector. The magnitudes of the photodetector output at the various angular deflections of the prism give the spectral content of the light beam.
Limitations of the sensitivity and accuracy of the prism spectrum analyzer have led to more sophisticated optical spectrum analyzers. Some of these have used a diffraction grating. Others have been based on a scanning Michelson-interferometer. Still others have used an optical heterodyne arrangement employing a scanning laser local oscillator configured from a laser with a mechanically tuned external cavity.
A Michelson spectrum analyzer uses a mechanically moving mirror to generate an interference pattern from which a measure of coherence is obtained. A mathematical calculation known as the Fourier transform is then performed to obtain the frequency spectrum from this coherence measurement. When performed on a set of discrete measurements, this calculation is referred to as a DFT (digital Fourier transform). In practice, this calculation is carried out by a computer using an FFT (fast Fourier transform) algorithm.
All such instruments depend on high-precision mechanical motion of an optically reflective or transmissive element.
There has been a need for an optical spectrum analyzer that can provide high accuracy and good signal-to-noise performance with no moving parts.